A spark plug is used for ignition in an internal combustion engine such as gasoline engine. In general, the spark plug has a center electrode and a ground electrode with a gap for spark discharge (called “discharge gap”) defined therebetween. In order to ensure good ignition performance, the center electrode usually includes a small-diameter portion located on a front end side (discharge gap side) thereof, a large-diameter portion located in rear of the small-diameter portion and made larger in diameter than the small-diameter portion and a connection portion connecting the small-diameter portion and the large-diameter portion to each other.
It has been proposed to provide the spark plug by laser welding an electrode tip, which is made of an alloy containing a noble metal (such as platinum, iridium, ruthenium or rhodium) of high resistance to spark wear and oxidation as a main component (hereinafter referred to as “noble metal tip”), to a front end (spark discharge region) of the small-diameter portion of the center electrode (see, for example, Japanese Laid-Open Patent Publication No. H6-36856; Japanese Laid-Open Patent Publication No. H3-176978; Japanese Laid-Open Patent Publication No. 2004-207219; Japanese Laid-Open Patent Publication No. 2005-150011; Japanese Laid-Open Patent Publication No. 2011-34826; and Japanese Laid-Open Patent Publication No. 2000-208235).
In the internal combustion engine, a higher compression ratio has been pursued in order to achieve not only high fuel efficiency by improvement of ignition performance but also high power output that runs counter to clean exhaust emission. There is however a problem that the risk of breakage of the center electrode increases due to increases in vibration and combustion pressure when the compression ratio of the internal combustion engine becomes high. In particular, the vibration of the center electrode is large so that there is a higher risk of breakage of the center electrode in the case where the large-diameter portion of the center electrode is relatively small in diameter.
In the case where the center electrode is constituted by the small-diameter portion with the noble metal tip joined thereto, the connection portion and the large-diameter portion as mentioned above, breakage is likely occur at a boundary region between the small-diameter portion and the connection portion of the center electrode. It is thus assumed that the breakage resistance of the center electrode will be improved by decreasing the axial length of the part of the center electrode situated in front of the boundary region, that is, the axial length of the part of the center electrode constituted by the small-diameter portion and the noble metal tip joined thereto. However, the ignition performance of the spark plug in the internal combustion engine deteriorates with decrease in the axial length of the part constituted by the small-diameter portion and the noble metal tip. In this way, there is a problem of achieving compatibility between improvement of the ignition performance of the spark plug and improvement of the breakage resistance of the center electrode.